The present invention relates to an image display device, and more particularly to an image display device of a field emission type.
Recently, attention has been directed to FEDs (Field Emission Displays), as a flat image display device. Many researches have also been conducted as to FED drive circuits. An active matrix system using active elements, for example, disclosed in Japanese patent publication No. 2656843 (patent document 1), is well known FED drive circuit.
An active matrix system, shown in FIG. 5, includes a thin-film transistor (TFT) section 1, a cathode section (FEC section), which has cone-shape emitters 16, a cathode electrode 15 connected to the cone-shape emitters 16, a gate electrode 13 having a large number of holes 13a, and an anode electrode 3 acting as a display substrate and having a surface on which a fluorescent substance layer 5 is coated. The thin-film transistor section 1 includes transistors Tr1 and Tr2. A drain 8 of the transistor Tr1 is connected to the emitters 16 via the cathode electrode 15. A gate 11 of the transistor Tr1 is connected to a source 7a of the transistor Tr2. A capacitor 12 is connected to the gate 11 of the transistor Tr1, a scanning signal is applied to a gate 11a of the transistor Tr2, and a clear signal or a display signal is selectively input to a drain 8a. This structure allows current magnitude to be controlled which flows from the drain 8 to the source 7, more specifically, which flows from the gate electrode 13 to the emitter 16 adjacent to the gate electrode 13 due to field electron emission. Although a plurality of emitters 16 are connected to each cathode electrode 15 in FIG. 5, a single emitter 16 may be connected to each cathode electrode.
The TFT array, which is formed of a plurality of thin-film transistors having the same configuration as the thin-film transistor section 1 formed on a substrate, is selected each array driving column in time sharing manner. At the same time, a matrix drive is carried out in sync with the time sharing operation to supply a display signal to each column in the array. Since each thin-film transistor section 1 in the TFT array is connected to each FEC array formed of a plurality of FEC sections having the same configuration as the FEC section, a capacitor voltage of a specific thin-film transistor section is selectively updated. Electrons are emitted due to field electron emission according to the voltage of the capacitor. In the operation, since the other capacitors in the TFT array maintain a present voltage until the voltage of the capacitors are next updated, electrons are continuously emitted from each FEC section during the maintenance of present voltage according to the voltage of each capacitor. In this case, the voltage of the gate electrode 13 is kept at a higher fixed level than that of the cathode section (FEC section).
A fluorescent substance layer 5 is coated over one or plural anode electrodes formed on a display substrate and an anode voltage is applied to the anode electrodes. Electrons emitted from each FEC section impinge on an opposed portion of the fluorescent substance layer to generate luminescence. The opposed portion of the fluorescent substance layer continues to produce luminescence at the same brightness until the capacitor voltage is next updated. A luminous time ratio (duty ratio) is to be approximately 1 so that a high intensity luminescence can be realized.
FIG. 6 shows a structure of the thin-film transistor section 1 and the cathode section. A cross section diagram of the transistor Tr1, which is a portion of the thin-film transistor 1 formed on a substrate, is shown on the left side of FIG. 6. The emitters 16 and the cathode electrode 15 connected to the emitters 16 in the cathode section are shown on the right side of FIG. 6. The source 7 and the drain 8 are formed on a glass cathode substrate 6 made of an electrical insulating material. A polycrystalline silicon semiconductor layer 9 is coated to bridge the source 7 and the drain 8. A gate insulating film 10 such as SiO2 is deposited on the semiconductor layer 9 to form a gate 11. Thus, a transistor Tr1 is formed. A lead for the gate insulating film 10 and a lead for the drain 8 extend to the FEC section on the cathode substrate 6 so that the cathode electrode 15 is formed. The emitters 16 are connected to the cathode electrode 15. The lead for the source 7 is grounded (not shown). The lead for the source 7 and the lead for the gate 11 are laminated via an insulating layer, whereby a capacitor 12 is formed in this area. The lead for the gate 11 is connected to the source 7a of the transistor Tr2 via a lead line.
As described above, the active matrix system known in the art is to adapt such a matrix driving method that each column of the TFT array is selected in time sharing manner and a display signal as a capacitor charging voltage is applied to each row in the array while a constant dc voltage is applied to a gate electrode in the configuration including TFT arrays acting as active elements.
In the active matrix system, the luminous intensity of the fluorescent substance layer depends on the voltage of each capacitor in principle as described above. However, when an overall brightness of an image displayed on a flat image display device or a brightness of a partial area of an image divided into several sections is required to change according to an ambient environment, it has been difficult to establish a condition to change the brightness only without significantly changing hue and contrast, because voltages applied on respective capacitors must be controlled depending on the overall brightness, which makes such control to be very difficult.